Various designs for conducting the combustion air in Stirling engines have been proposed. Known Stirling engines (see for instance the U.S. Pat. No. 3,811,272, the British Patent No. 1,394,033, the British patent application No. 15386/86 and the Swedish application No. 7301058-9) were so designed that the combustion air was fed via a single continuous duct between the air preheater and the burner. Such ducts were made of straight sheet metal pipes with flat walls or, in the case of small engines with only a single burner and a round arrangement of the heater pipes and the air preheater, by concentrically arranged conical sheet metal shrouds. In the case of Stirling engines with heater spaces having a large size and more especially in the case of engines with a number of serially arranged heater spaces, the high thermal load tends to bulging and kinking or creasing of such flat or slightly dished walls of the ducts for the combustion air. This in turn leads to poor aerodynamics at the surface of the sheet metal, this leading to a reduced cooling of the heat exchange surfaces by the combustion air to be preheated, and ultimately to overheating or even fusing of the sheet metal walling.
In the case of high air flow rates such sheet metal ducts have to be stiffened by ribs, which greatly increases the price of the duct system.